Vacuum tube amplifier



United States Patent VACUUM TUBE AMPLIFIER Ernii Winter, Budapest, Hungary, assignor to Egyesult Izzolampa es Villamossagi Reszvenytarsasag, Budapest, Hungary, :1 Hungarian enterprise Application February 28, 1956, Serial No. 568,400

Claims priority, application Hungary March 8, 1955 Claims. (Cl. 315-16) This invention relates to vacuum amplifier tubes and particularly to such tubes operating with D.-C. electron currents of the order of magnitude of one microampere and having a low noise level and a steep slope in the characteristic showing the relationship between platecurrent and control voltage.

It is a disadvantage of the amplifier tubes according to prior art arrangements that they are not adapted to the amplification of signals of the order of magnitude of about one microvolt. Assuming a slope of the characteristic of 3 ma./v., the amplitude of the D.-C. component of the plate current surpasses the amplitude of the A.-C. component by several orders of magnitude with voltages of the order of magnitude of one microvolt to be amplified, D.-C. components of this magnitude considerably increase the noise-level of the tube. Therefore it has been practically impossible to amplify low voltages with a satisfactory gain, the amplitude of the useful signal being a small fraction of the amplitude of the plate current this resulting in an increase of the signal-to-noise ratio of the tube.

The signal-to-noise ratio of the tube would be materially improved with the D.-C. and A.-C. components of the plate current being of the same order of magnitude. This condition has been fulfilled in various types of electron multipliers, but these devices were only adapted to the amplification of photo-signals.

In multi-grid electron discharge tubes, disclosed for example in the Hungarian Patent No. 111,018 the slope of the characteristic has been materially improved by means of a potential minimum referred to as a virtual cathode and set up before the third grid, the so-called suppressor grid, by means of the negative bias voltage of said third grid. Variations of the voltage of the suppressor grid afiect the slope of the characteristic and therewith the gain. According to prior art arrangements virtual cathodes have been set up before one or two grids of electron discharge tubes operating with electron beams. In all these arrangements either the distance between the grids setting up the virtual cathode was not a homogeneous value due to the oval or arcuated shape of the grid, causing an intermittent contact between the virtual cathode and said grid, or generally the virtual.

cathode was set up far off before the grid.

The principal object of the present invention is to provide an improved vacuum amplifier tube especially suited for amplifying signals of the order of magnitude of one microvolt.

Another object of the invention is to provide an improved vacuum amplifier tube adapted to the amplification of very low voltages of the said order of magnitude whereby the noise-level of the tube is low.

Since another object of the invention is to provide an improved vacuum amplifier tube performing a steep slope even with a D.-C. electron current of the order of magnitude of one microampere.

To attain this end the invention contemplates the incorporation of a number of distinct features, some of 2,881,355 Patented Apr. 7, 1959 ice which may have application in other types of vacuum discharge devices.

In accordance with the invention the vacuum tube amplifier for the amplification of signals of the order of magnitude of one microvolt comprises an evacuated envelope, a thermionic or field emission cathode, an anode, a set of electrodes for focusing the electron current leaving said cathode and located between said cathode and at least one further electrode located between the last 0 focusing electrode and said anode, the geometrical dimensions of the cross-sections of said electron current, its density and the distance between the last focusing electrode and at least one of said further electrodes being chosen so as to set up a virtual cathode upon at least one of said further electrodes. Hereby the electron current flow leaving the cathode is of the order of magnitude of about one microampere.

It has been found that in order to attain a very steep slope per ma. anode current the virtual cathode is in its whole extent to be set up upon at least one of said further electrodes or at least in its close proximity, acting practically as a potential minimum set up by the space charge upon said further electrode, so that this electrode may be regarded as the virtual source of the electrons flowing to the anode. As a result of this virtual cathode the slope of the characteristic in the plane of the electrode acting as a virtual cathode will ensure an advantageous actual performance even for 1 ma. plate current. With this small electron D.-C. current the noiselevel of the tube is low.

The forming of the virtual cathode depends upon the value of electron density, the velosity of the electrons, the distance the electron travels along from the last focusing electrode to the position of the virtual cathode and the shape and structure of the electrode, for example a grid, upon which the virtual cathode is set up. As these factors may be varied, in one preferred embodiment of the invention the virtual cathode may be formed upon the control grid of the tube. Thus a low noise-level of the tube and amplification of low voltages with low electron current may be realized. Due to the short distance between the potential minimum set up by the space charge, i.e. the virtual cathode and the control-grid, the distance along which the electron is in flight between said two electrodes, the tube will be adapted to amplify even ultrahigh frequency signals.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention will be best understood by reference to the following description taken in connection with the accompanying drawings, in which:

Fig. 1 is a diagrammatic view exemplifying the manner in which a device according to my invention is built up and operated.

Figs. 24 show three different modifications of a grid- -electrode, adapted to be applied in a tube according to Fig. 1. scale.

Fig. 5 shows the relationship between plate-current and control-voltage, the current being plotted in microamperes, the voltage of the control-grid in microvolts. U and U are parameters.

The same reference numbers designate the same components in all figures.

Referring now to Fig. 1 of the drawing, 4 denotes the sealed envelope of a discharge tube. Mounted within and adjacent to the bottom of the envelope is a cathode 5, and adjacent to the opposite end of the envelope an anode 6. 11 denotes a control grid. Between the cathode 5 and the control grid 11 there are provided four focusing electrodes 7, 8, 9 and 10 adapted to afiect the electrons The grid electrodes are shown on an enlarged leaving. the cathode. so, as. to. form. a well-defined. sharp electron beam of high electron density and thus set up a virtual cathode upon the control-grid 11, after travelling along a short path. even, wi t-h1an electron current of the order of magnitude of one microampere.

Thefocusing electrodes 7-10, are, preferably made of suitable metal plates; and are each provided with, a single tiny slit 29. The electrons leaving the cathode 5 pass through; said slits, 29 in the focusingelectrodes, except those making a small anglewith the axis. of the electron beam., The slits 29 in, the electrodes 7-10 preferably register with each other especially the slits 29 in the electrodes 7 and 8. Suitable accelerating potentials are impressed p nthe fQc sinae1ec rdes7-1fl by meansof a-va esupplv a d ano n iome er 3.7 n ps 38 @439, Tan 39 is-pla ed on a andard. r si -er; 40 c nnected, to...tl ,1 c voltage; source. 41 supp y g he ca hode 5.

e negativ -bi voltageof hefirst focus ng ele trode; 7 ,rnay be adjusted. The sli ;ble tap is,.placed on the resistor 37 to; provide, variable. voltagefor the focusing electrode 9. In the preferredjemhodiment of .the, invention shown in Fig. 1 there is impressed a negative potential upon the electrode 7 and positive potentials upon the electrodes 8, 9 and 10. The positive potential of the electrode 10 is higher than that impressed upon the electrode 8. V The positive potential of the electrode 9 may be varied by means of the movable tap 12 and shall in any case be lower than the potential corresponding to the position of this electrode.

The efficiency of concentration of focusing of the electron beam leaving the cathode 5 may. be increased by means of deflection-plates or pairs of such plates setting up electrostatic deflecting fields, these plates being arranged in addition to the focusing electrodes 7-10, prefferably between the electrodes and 11.

The control grid '11, preferably made of a metal plate, has but one tiny slit 35 and the electron beam leaving the cathode 5 is by means of said focusing electrodes 7-10 concentrated to. this slit 35. The setting up of a virtual cathode, is backed up by the higher positive potential of the focusing electrode 10.

It has been stated above that the forming of the virtual cathode depends-besides other values-upon the density of the electrons. The higher this density the more easily the virtual cathode is set up. It depends, further, upon the velocity of the electrons, in other words upon the potentials of'the focusing electrodes, especially the potentials of the first focusing electrode 7 and the last foc'us'i'ng'elctrode 10. 'The greater the potential difference between the cathode 5 and thefocusing electrode 7, the

feat fthenumber of; the electronsv reaching the controlfgrid'fllfand the. greaterthe plate current. The velocity "of thefele'ctrons between the cathod '5 and the control grid 11 is similarlyaffected bythe focusing electrodes 8-1 0, especially by the value of the positive potential of the last focusing electrode 10. The lower the velocity of the electrons, the more easily the virtual cathode is set up. The distance between the last focusing electrode 10 and the control-grid 11 constitutes another important value. In'gener al the virtual cathode is formed the, more easily the shorter this distance is and the smaller the opening 35, in the control-grid 11. Still another important value is constituted by the bias voltage of this grid.

The potential minimum setup by the space charge and referred to as a virtual cathode, because it may be regarded as a virtual source of the electrons flowing to the plate, i.'e. in the discharge tube, according to the invention, to theanode 6, forms at a critical valuel 'of'the electrbneurrent leaving the cathode:

the value of K is a constant:

f heh ol ag s. su t u e n vo n r s t n lliamperes. F is the cross-section of the electron beam and D the distance between theaccelerator grid and the plate, i.e. in the tube according to Fig. l the distance between the last focusing electrode 10 as an accelerator grid and the control grid 11 as a plate. v is the eifective voltage of the last focusing electrode 10 and Vaeff the elfective voltage of the control grid 11 on the hole or gap of which the virtual cathode is set up. (The elfective voltage of an electrode is the value of the voltages of all other electrodes located behind said one, reduced to the position of said one cathode.) I

Let V =50 volts, V =l00 volts, D=l cm. and F=1 cm. then a control Voltage on the grid 11 of only 9-16 volts effective and the cross-section of the, electron beam measuring 0.001 cm), the virtual cathode will be set up with an electron current of l microampere. Due to the fact'of-D beinga quadratic factor its variation highly affects the value oflg p H Since the electron beam is by means of the-focusing electrodes 7-10. concentrated upon the tiny slit or gap 35 in the control grid 11 with an electron beam of small cross section and the electrode potentials chosen adequately, the virtual cathode will be set up in this slit or gap and will act as a source of the electrons flowing towards the plate 6. The amplitude of the AC. component of the plate current will be equal to the amplitude of the DC component of said plate current, and the DC. component itself being of the order of magnitude of one microampere the noise-level of the tube will be lowered.

Since the distance between the virtual cathode and the control grid 11 is very short, practically equal to zero and the input attenuation is ofja small value, the amplitier tube according to the invention is well adapted to amplify even signals in the ultra high frequency range. Within the amplifier tube, according to the invention,

plane grids. In the embodiment shown in Fig. 1 or the drawing there are located two such further grids 12 and 13 between the control grid 11 and the plate 6. Upon the grid 12 there is impressed 'a positive potential from the potentiometer '37. The grid 13 is a suppressor grid. These'grids may also be made with one single slit or gap, like the control grid 11and the electrodes shown in Figs. 2-4 and described in the following part of this specification.

' In Figs. 2-4 there are shown three modified forms of the control grid 11 of the amplifier tube according to the invention.

The control grid shown in Fig. 2 consists of two plane metal sheets 18 and 19,- ri'gidly connected to each other by {means'of strips-"20 and 21 in such a wa that there "remai'nsa tiny slit 22 surrounded by said members 18-21.

The slit 22 has been drawn on an enlarged scale. "Its "dimensions, i.e. its cros's-'section is preferably of the order plane sheets 23 and 24 rigidly connectedto each other by means of strips 25 and 26 in such a way that there remains a narrow gap 27 of an oblong form. This gap is also drawn on an enlarged scale. Its dimensions are preferably equal to the dimensions of the electron beam concentrated by the focusing electrodes 4-7 upon said gap 27. This gap may have for instance a height of'l mm. and a width of 0.1 mm., or a height of 10 mm. and a width of 0.01 mm.

The modified embodiment of the grid shown in Fig. 4 differs from that shown in Fig. 3 only in the gap between the plates 28 and 29 and the linking strips 30 and 31, said gap having a somewhat greater width. In this gap there is provided a set of very thin wires 32, preferably made of tungsten and each having a diameter of about a few microns. The wires 32 may be arranged longitudinally in the gap as shown in Fig. 4, or crosswise, or longitudinally and crosswise. They are provided in the plane of the electrode and span the gap.

The control grid 11 to be applied in the tube according to Fig. 1 may also be made of one single piece of sheet metal and be provided with a slit or gap in any known manner.

The material, the various electrodes of the tube according to the invention are made of, may be any of the metals or alloys commonly used for electrodes in vacuum tubes, for instance molybdenum or tungsten. The wires 32 of the grid shown in Fig. 4 are preferably coated with a thin layer of gold.

Fig. shows the relationship between the plate current and the voltage of the control grid. The lower curve shows the slope for a voltage of 2 volts of the first focusing electrode 7, the higher curve for a voltage of -1 volt of the same focusing electrode. According to theories prevailing up to now the virtual cathode, i.e. the potential minimum of the space charge is set up with a certain dis continuity. The diagram in Fig. 5 shows no such phenomenon. Discontinuity would result with a perfectly dense cathode, without any slit or gap in it. I have found that the virtual cathode is first set up on the two edges of the gap of the electrode 11, for instance on the longitudinal edges of the gap of the grids shown in Figs. 3 and 4 and then continually expands towards the middle of said gap. The setting up of the virtual cathode is accelerated by means of the thin wires 32 provided in said gap.

The gap 35 in the grid 11 may, however, be of any suitable form, such as a quad, a circular hole, or the like.

Although only one embodiment of the amplifier tube according to the invention has been described, it will be understood by those skilled in the art that this specific form may be modified without departing from the spirit of my invention and I desire to avail myself of such modifications as come within the scope of the appended claims.

I claim as my invention:

1. A vacuum tube amplifier for the amplification of signals of the order of magnitude of one microvolt, comprising in combination a sealed envelope, a cathode, an anode, a set of electrodes for focusing the electron beam leaving said cathode and located between said cathode and at least one further electrode located between the last focusing electrode and said anode, the geometrical dimensions of the cross-section of said electron beam, its current density and the distance between the last of said focusing electrodes and at least one of said further electrodes meeting the condition in order to set up a virtual cathode upon at least one of said further electrodes in which I denotes the critical value of the electron current leaving the cathode; K is a constant corresponding to the equation F is the cross-section of the electron beam; D is the dis tance between the accelerator grid and the plate; V828 is the effective voltage of the last focusing electrode;

V is the effective voltage of the further electrode acting as a control grid on the hole on which the virtual cathode is set up.

2. A vacuum tube amplifier according to claim 1, comprising a control grid located between the cathode and anode and made of sheet metal having one single slit in it for electrons passing said grid and a set of electrodes located between said cathode and said grid.

3. A vacuum tube amplifier according to claim 1, comprising at least one further electrode made of two dense metal sheets rigidly connected to each other and forming one single opening between themselves for electrons passing said electrode.

4. A vacuum tube amplifier according to claim 1, comprising at least one further electrode made of sheet metal and having one single slit in it, the cross-section of said slit being of the same order of magnitude as the crosssection of said electron beam concentrated by said focusing electrodes upon said hole.

5. A vacuum tube amplifier according to claim 1, comprising at least one further electrode made of sheet metal and having one single opening in it, a set of thin wires provided in the plane of said electrode and spanning the opening, the cross-section of said opening being of the same order of magnitude as the cross-section of said electron beam concentrated by said focusing electrodes upon said opening.

6. A vacuum tube amplifier according to claim 1, comprising four electrodes for focusing the electron beam leaving the cathode.

7. A vacuum tube amplifier as claimed in claim 1, comprising a control grid and at least one further grid electrode located between said control grid and the anode.

8. A vacuum tube amplifier system according to claim 1, comprising at least one grid electrode between the last focusing electrode and the anode, a source of voltage, a voltage divider connected to said voltage-source for impressing predetermined accelerating voltages on said focusing electrodes, the voltage impressed on the first focusing electrode being negative, the voltages im pressed on the following three focusing electrodes being positive and at least one of said grid electrodes being connected to the input of the system.

9. A vacuum tube amplifier system as claimed in claim 1, comprising between the last focusing electrode and the anode a source of voltage, a voltage divider connected to said voltage-source for impressing predetermined accelerating voltages on the focusing electrodes, the positive voltage impressed on the fourth focusing electrode being higher than the positive voltage impressed on the second focusing electrode, and an adjustable voltage divider connection for impressing an adjustable positive voltage on the third focusing electrode, this positive voltage being lower than the one corresponding to the place of the third focusing electrode.

10. A vacuum tube amplifier system as claimed in claim 1, comprising a control grid, an anode, a set of electrodes for focusing the electron beam leaving the cathode and located between the cathode and the control grid, two further grid electrodes located between said control grid and said anode; a source of voltage, a voltage divider connected to said voltage-source for impressing predetermined accelerating voltages on said focusing electrodes, the voltage impressed on the first focusing electrode being negative, the voltages impressed on the next following three focusing electrodes being trode located between said control "grid and: the

7 positive, the-voltage impre sed first ya and adjacent to the control grjd being positive and the voltage impressed; up'cifl the; secondgxid electrode located between the control grid and the anode and adjacent to the anode beingme'gative, the c'ont-x'olwgrid being connected to the inpub'of the system.

Reieliences; Gited in th'e file of thi patent IQ 71 "i-P 'Q-E -c 2 Ll wellyn. V-.---,--.- p 247 Adler Sept. 23, 1952 

